Add state of an MR upon merge to audit event

Problem to solve

We already have the merge_request_approval_operation and merge_request_merged_by_project_bot audit event types to track when a MR is approved and merged https://docs.gitlab.com/ee/administration/audit_event_types.html#code-review-workflow. We also have violations to indicate when an MR is approved and a rule has been violated https://docs.gitlab.com/ee/user/compliance/compliance_center/#violation-types. But this information is all separated and makes it hard for users to get a picture of MR at merge.

Proposal

Add more information to the details field for the MR merge event, such as:

• MR title
• MR description (or summary)
• How many approvals required
• How many approvals granted
• Which users approved
• Did committers approve?
• Did authors approve?
• Date/time of merge
• List of SHAs from Commit history